The intestinal epithelium is a highly regenerative tissue that undergoes rapid turnover during homeostasis. Failure to effectively regenerate the epithelium after injury such as the enteropathy associated with radiation therapy or ischemia-reperfusion associated with an array of surgical procedures can result in breakdown of the intestinal barrier and bacterial translocation into the bloodstream, leading to cycles of inflammation and further epithelial cell death. Intestinal homeostasis and regeneration after injury are governed by a hierarchical stem cell system. This system includes dormant reserve intestinal stem cells (ISCs) that give rise to active, rapidly cycling crypt base columnar stem cells (CBCs) that fuel normal intestinal homeostasis. The active CBCs periodically undergo exhaustion and are replaced from the reserve ISC compartment. In the context of acute injury such as radiation exposure, active CBCs are destroyed, driving the reserve stem cells to become activated en masse. These reserve ISCs divide to replenish the CBC population, which subsequently regenerates the epithelium and restores barrier function. In the face of severe acute injury, inefficient epithelial regeneration can ultimately lead to high morbidity and mortaliy. Thus, the goal of this proposal is to elucidate the molecular mechanisms underlying reserve stem cell-driven regeneration of the intestinal epithelium in order to manipulate these mechanisms to either enhance regeneration after acute intestinal injury, or to prophylactically mitigate the effects of injury in advance. Until recently, the molecular mechanisms governing reserve stem cell activation have remained elusive. We present preliminary data identifying the first molecular mechanism known to govern reserve stem cell activation: the Msi-mTORC1 axis. We demonstrate that stimulation of the Msi-mTORC1 axis is sufficient to drive reserve ISCs out of quiescence and into the cell cycle, and that loss of function of the Msi family of RNA binding proteins abolishes the regenerative capacity of the epithelium. Our central hypothesis is that activation of the Msi-mTORC1 axis is both necessary and sufficient to drive reserve ISC activation and epithelial regeneration in response to injury. We will test 1) the fate of reserve ISCs upon Msi loss of function in response to injury in vivo, 2) the functional contribution of Msi RNA binding targets to mTORC1 activation, reserve ISC activation, and epithelial regeneration, and 3) how dietary modulation effects mTORC1 activity (mTORC1 is a major nutrient sensing-complex), and subsequently the regenerative response of reserve ISC to injury. These studies will identify novel points for therapeutic manipulation of reserve ISC activity to either promote efficient epithelial regeneration after injury, or to prophylactically protect the epithelium in advance of injury.